Considerable mystery but little actual detecting as we expect it. More of a thriller, in fact. Well written and fast-moving.
The best thing about this is the depiction of the heroine with an odd name, but she isn't good enough to single-handedly overcome Hume's fantastical plotting.
French authors, especially of the era depicted, have rather bizarre notions of how society ought to operate, including the favoring of criminals over the forces of law. Crystal Stopper is one more example of this, and requires a great suspension of disbelief as well. I feel I did well by getting a third of the way into the story before giving up on LeBlanc.
Quite an exciting and well-written mystery until the denouement, which I consider a great disappointment.